1. Field of the Invention
The invention relates to an illumination system for an electrooptic colour display screen and especially a liquid-crystal trichrome projector. The object of this system is to improve the colorimetric performance of a liquid-crystal projector.
2. Discussion of the Background
In a trichrome projector it is advantageous to obtain trichromatic ranges which are as saturated as possible in order to increase the reproduced colour shades of the image and to approach as far as possible the colorimetric content of the picture. It is therefore necessary first to choose the lighting sources judiciously.
However, one of the main limitations of liquid-crystal projection devices is provided by the illumination device. This limitation stems partly from the limited choice of white sources compatible with this application. This is because white sources must simultaneously satisfy the criteria of high light efficiency, of stability of their colorimetry and of a long lifetime, as well as meeting cost requirements. It appears that currently arc lamps of the metal-halide type constitute the best compromise between these criteria.
These arc lamps are more particularly developed for cinema projection exhibiting a colorimetry suitable for this application since its chromatic coordinates x/y are very close to those of the television standard reference xe2x80x9cwhitexe2x80x9d.
In order to be used in a liquid-crystal projector, the light emitted by these arc lamps must undergo chromatic separation so as to obtain various beams having different wavelength ranges.
In a known manner, three beams, in the green, the blue and the red, are extracted from the beam emitted by the lamp.
This chromatic separation may be performed using dichroic mirrors, as is described in French Patent No. 92/15382, so as to obtain separate mono-chromatic beams each illuminating a liquid-crystal screen. The light emitted by the various screens is then mixed so as to superimpose the images produced by these screens.
Chromatic separation may also be performed by means of a spatio-chromatic separation system which separates the various wavelength ranges into angularly different directions. A single liquid-crystal screen is then used in which each image element (dot) possesses as many elementary display elements (pixels) as there are wavelength ranges. Typically, a dot therefore possesses one image element for red, one for blue and one for green. French Patent Application No. 93/08470 describes one embodiment of such a system.
However, the reproduction of a colour image, by means of image elements each emitting a primary colour, requires that each primary be suitably saturated, that is to say that the wavelengths which it contains form a primary colour close to the TV standard. In addition, it is necessary that the energy level of each primary be matched. This is what is called colorimetric balance. French Patent Application No. 92/15382 describes a device in which a filter is provided which allows complete elimination or almost complete elimination (to an extent up to or exceeding 84%) of certain undesirable wavelengths having a contaminating effect, in the colorimetric sense, on the saturation of a primary. For example, this pertains to elimination, in some lamps, of a very intense wavelength close to 570 nm, corresponding to yellow. This device thus allows elimination of wavelengths prejudicial to the purity of the primary wavelengths and therefore contributes to obtaining good saturation of these primaries.
However, it appears that the elimination of certain wavelengths decreases the energy level of the primary band to which they belong. Likewise, it is observed that the energy level of certain primaries should be reduced compared to the energy level of the other primaries (colorimetric balance) without correspondingly eliminating wavelengths. For example, in the beam emitted by some lamps, it may be advantageous to reduce the light intensity at the wavelength of yellow in order to decrease the overall intensity of the green primary.
The invention provides a solution to such a problem.
The invention therefore relates to an illumination system for an electrooptic colour display screen, comprising:
a polychrome light source;
a chromatic separation system receiving a polychrome light beam coming from the source and transmitting several illuminating light beams having different wavelength ranges and in different directions;
an electrooptic display screen, each image element or dot having as many display elements (pixels) as there are illuminating light beams;
characterized in that it includes at least one chromatic modulation device lying between the source and the display screen, this device being tuned to a particular wavelength and transmitting a defined part of the light energy which it receives at this wavelength to the display screen.